$ { {1} \times \left[\begin{array}{rr}{4} & {0} \\ {-1} & {3}\end{array}\right] = {?} } $
To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{rr}{1}\times{4} & {1}\times{0} \\ {1}\times{-1} & {1}\times{3}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rr}{4} & {0} \\ {-1} & {3}\end{array}\right]}$